Method for selective deposition of refractory metals on silicon substrates and device formed thereby

ABSTRACT

Selective deposition of a refractory metal on a silicon substrate utilizing high temperature and a silane reduction process in which the flow rate ratio of silane to refractory metal halide gas is less than one. In a second embodiment, an additional layer of the refractory metal is deposited utilizing a hydrogen reduction of the metal halide gas at very high temperatures. In both embodiments, a refractory metal barrier layer may be provided by forming a self-aligned refractory metal silicide layer. Alternatively, a two layer self-aligned barrier is formed of a refractory metal silicide lower layer and a refractory metal nitride upper layer and the refractory metal is selectively deposited on the metal nitride.

This application is a continuation-in-part application of co-pendingU.S. application Ser. No. 294,014, filed Jan. 6, 1989 by the sameinventors herein and assigned to the same assignee as the presentinvention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to deposition techniques for forming metalregions on semiconductor substrates, and more particularly, to theselective deposition of refractory metals on a refractory metal nitrideand/or silicide barrier layer and the semiconductor devices formedthereby.

2. Description of the Prior Art

The continued miniaturization of integrated circuits has brought aboutan increasing need to reduce the resistance in the source-drain-gate andcontact metallurgy. In recent years, much effort has been focused on theuse of metal silicides to fulfill this need. However, as devicedimensions become even smaller, both vertically and horizontally,silicides lose their attractiveness. The intrinsic resistivity of thesilicides is high compared to metals, while the formation ofself-aligned silicides consumes silicon in proportion to the thicknessof the silicide that is formed. This consumption usually leads tojunction leakage which is intolerable. U.S. Pat. 4,701,349 is directedto a method for depositing a self-aligned interconnect comprised of alayer of titanium nitride and a layer of titanium silicide. The sheetresistance of the titanium nitride layer is on the order of 15 ohm/sq.

Refractory metals have been investigated as possible alternatives tosilicides. The low resistances and relatively high temperature stabilitymakes the refractory metals attractive. In addition, the recentdevelopment of selective chemical vapor deposition (CVD) processes, havemade tungsten and molybdenum prime candidates to replace silicides. Inaccordance with the CVD technique, tungsten (W) is deposited on thesurface areas by placing the substrate in a CVD reactor and heating thesubstrate. Tungsten hexafluoride (WF₆) and an inert carrier gas such asargon (Ar) or nitrogen (N₂) are then fed into the reactor and thetungsten hexafluoride will react with the silicon in accordance with thefollowing:

    2WF.sub.6 +3Si→2W+3SiF.sub.4

The deposition of tungsten will stop and in order to deposit additionalmaterial, hydrogen (H₂) is added to the tungsten hexafluoride andcarrier gas. The tungsten hexafluoride will react with the hydrogen todeposit the desired additional tungsten in accordance with thefollowing:

    WF.sub.6 +3H.sub.2 →W+6HF

The use of the above hydrogen reduction process to selectively deposittungsten for VLSI applications has been limited by problems inherent inthe deposition process. The problems include unpredictable, deeptungsten penetration into silicon regions (worm holes) due to attack ofHF liberated in the reaction. In addition, encoachment problems occurdue to the tungsten penetrating along nearby silicon dioxide/siliconinterfaces. Furthermore, problems result from the poor adhesion of thetungsten to silicon.

Kotani, et al., IEDM (1987 IEEE) disclose a CVD process for theselective deposition of tungsten utilizing silane (SiH₄) reduction.Kotani, et al. use a low formation of undesirable silicide peaks whichincrease the resistivity of the tungsten layer to about 20 μohm-cm.

The use of barrier layers to enhance adhesion and lower contactresistance for tungsten films has been recently demonstrated by Brodsky,et al., IBM Tech. Bull., Feb. 1986. A titanium nitride layer wasdeposited by sputtering prior to the tungsten deposition by sputtering,electron beam evaporation or low pressure CVD. However, adhesion andresistance difficulties have been encountered in the deposition ofselective tungsten on these barriers.

SUMMARY OF THE INVENTION

The present invention is directed to a method for selectively depositinga refractory metal layer on exposed silicon surfaces of a siliconsubstrate utilizing the silane reduction of a refractory metal halide.The deposition is performed in a high temperature CVD system in whichthe silane/refractory metal hexafluoride gas flow rate ratio is lessthan one. The high temperatures and the low flow rate ratio result inthe formation of a refractory metal layer having a resistivity of lessthan 17 μohm-cm. The process is performed in a CVD deposition apparatusat temperatures in the range of 370° to 550° C. In an alternativeembodiment, after an initial layer of refractory metal is depositedutilizing the silane reduction of the refractory metal halide process,hydrogen is introduced into the deposition apparatus in place of silaneand the hydrogen reacts with the refractory metal halide to furtherselectively deposit refractory metal on the substrate. The additionalhydrogen reduction step is also performed at high temperatures, whichare typically above 500° C.

In the use of the process of the present invention for field effecttransistor (FET) metallization, it is preferred that a barrier layer beintroduced between the silicon and the refractory metal layer to preventencroachment. In this embodiment, a refractory metal barrier layer isdeposited prior to the deposition of the refractory metal layer. Thebarrier layer may be formed of a single layer of a refractory metalsilicide or it may be a two layer structure having a refractory metalsilicide as a lower layer and a refractory metal nitride as an upperlayer. The barrier layer or layers are deposited using a self-aligneddeposition process eliminating the need for photolithographic masks.Initially, the blanket layer of refractory metal is deposited on thesubstrate and the substrate is annealed to convert the metal to a metalsilicide where the metal is in contact with silicon. The unreacted metalis then removed using a selective chemical etchant. Thereafter, theupper surface of the metal silicide layer is converted to a metalnitride layer by annealing in an ammonia atmosphere with or withoutplasma. After formation of the barrier layer, the refractory metal layeris deposited as indicated above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing showing the cross-section of an integratedcircuit chip including the deposition of the refractory metal using themethod of the present invention.

FIG. 2 is a schematic drawing which illustrates a typical apparatus usedin the method of the present invention.

FIGS. 3a-3e are sectional views, each showing the production steps inaccordance with the method of the present invention for forming thebarrier layer and the refractory metal layer.

FIG. 4 is a plot of resistivity versus turret temperature of arefractory metal film formed in accordance with the method of thepresent invention.

FIG. 5 is a plot of contact resistance versus contact size of arefractory metal film found in accordance with the present invention.

FIG. 6 is a plot of stress versus turret temperature of a refractorymetal film formed in accordance with the method of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, a refractory metal isselectively deposited on a silicon substrate utilizing a hightemperature CVD process of silane reduction of the refractory metalhalide. In accordance with the inventive method, the ratio of thesilane/refractory metal halide flow rates is less than one. The methodof the invention may be utilized to deposit a refractory metal as acontact for a semiconductor device and as an interlevel viainterconnect.

FIG. 1 shows a device structure having the refractory metal contactsformed thereon. The device 10 is a MOS field effect transistor having asilicon substrate 12 and source and drain diffusion regions 14 and 16. Asilicon gate electrode 18 is formed by typical photolithographictechniques with a gate oxide layer 20 and sidewall silicon dioxidespacers 22 and 24. Recessed oxide isolation regions 26 and 28 separatethe structure 10 from other devices on the chip. The refractory metalcontact layer is selectively deposited on the source, drain and gateregions to form contacts 30, 32 and 34, respectively. The illustrativeembodiment shown in FIG. 1 includes barrier layers 36, 37 and 38disposed between the refractory metal layers and the underlying silicon.The barrier layers are formed by a self-aligned process to be describedin detail below.

The method of the present invention may be carried out in a typical CVDapparatus as shown in FIG. 2. The apparatus 40 includes a depositionchamber 42 having the substrate specimens 44 disposed on a turret 46. Inaccordance with the process for the silane reduction of the refractorymetal halide, the silane gas, SiH₄, is introduced into the chamber 42through ports 48 and the refractory metal halide gas, such as WF₆, isintroduced into the chamber 30 through ports 50. SiH₄ can be introducedin the chamber before or after the introduction of WF₆ and in eithercase the ratio of less than one can be maintained. Excitation isprovided by the rf generator 52 and an infrared sensor lamp assembly 54is also provided. A high vacuum valve 56, a throttle 58, a blower 60 anda pump 62 are provided to exhaust chamber 42 through port 64.

Refractory metals suitable for the formation of contacts and interlayerinterconnects include tungsten, molybdenum and tantalum. Suitablerefractory metal halides for the present method include pentachlorides,hexachlorides, pentafluorides and hexafluorides. In an illustrativeembodiment of the invention, tungsten is deposited by introducing silanegas and tungsten hexafluoride gas into the chamber 42 wherein the gaseswill react to deposit tungsten selectively on the exposed siliconsurfaces in accordance with the following:

    2WF.sub.6 +3SiH.sub.4 →2W+3SiF.sub.r +6H.sub.2

The silane reduction of tungsten hexafluoride provides a very highdeposition rate with excellent electrical characteristics. In accordancewith the method of the invention, the ratio of the flow rates of silaneto the WF₆ is less than one and preferably is approximately 0.2 to 0.6.In addition, the CVD deposition takes place at relatively high turrettemperatures in the range of 370° to 550° C. Preferably, the temperaturerange is between 430° to 480° C.

In the above embodiment, the entire thickness of the tungsten depositedon the silicon substrate is by the silane reduction process. In a secondembodiment, after depositing a layer of tungsten by the silane reductionprocess, H₂ is introduced into the deposition chamber 42 in place of thesilane through port 48. The hydrogen will react with the WF₆ to furtherselectively deposit tungsten in accordance with the following:

    WF.sub.6 +3H.sub.2 →W+6HF

The hydrogen reduction process is carried out at high turrettemperatures, typically above 500° C. and preferably at 550° C. Theformation of HF is not damaging to underlying layers because of theprotection provided by the initial layer of tungsten deposited by thesilane reduction process.

When the refractory metal halide is a pentahalide such as tantalumpentachloride the tantalum is selectively deposited on the exposedsilicon surfaces in accordance with the following:

    4TaCl.sub.5 +5SiH.sub.4 →4Ta+5SiCl.sub.4 +10H.sub.2

Upon introduction of hydrogen after depositing a layer of tantalum, thehydrogen will react with the TaCl₅ in accordance with the following:

    2TaCl.sub.5 +5H.sub.2 →2Ta+10HCl

In another embodiment of the invention, a self-aligned barrier layer issandwiched between the tungsten and the silicon. The barrier layer maytake the form of a refractory metal silicide alone, or in combinationwith a refractory metal nitride. In addition certain noble metalsilicides may also be used to form the barrier layer.

FIGS. 3a-3e show the production steps for the formation of the barrierlayer and subsequent tungsten deposition. As shown in FIG. 3a, asubstrate structure ready for formation of metal contacts includes asubstrate 70 having source and drain diffusion regions 72 and 74 and agate region 76 of polysilicon. The structure further includes gate oxideregion 78 and gate sidewall spacer silicon dioxide regions 80 and 82.Recessed oxide isolation regions 84 and 86 are also provided. In thefirst step of the process of forming a barrier layer, a thin blanketlayer 88 of pure refractory metal is deposited on the oxide and siliconwindows.

Typical refractory metals that can be utilized to form the barrier layerare titanium (Ti), niobium, zirconium, chromium, hafnium. The typicalnoble metals are cobalt and platinum. The preferred choice for forming arefractory metal silicide is titanium. TiSi₂ has the lowest resistivityamong the refractory metal silicides. In addition, due to its oxygengettering capability, Ti is a good oxide reducing agent. Ti can dissolvesilicon native oxide and can consistently form TiSi₂ upon annealingwithout much difficulty. It has been shown that subsequent contactintegrity can be preserved if a barrier layer such as TiSi₂ is placedbetween the tungsten and the silicon to stop silicon diffusion into thecontact.

In addition, a composite silicide layer may be formed by combining tworefractory metals. For example, one of the above mentioned refractorymetals may be combined with any other refractory metal, such astungsten, to form a composite silicide, Ti-W-Si₂. Any other combinationof refractory metals may be used to form the silicide barrier layer.

For many applications, a layer of TiSi₂ may be a sufficient barrier toallow the subsequent selective deposition of tungsten to occur withoutthe encroachment and penetration problems described above. However, fortitanium silicide, it has been found that an additional barrier layer oftitanium nitride be disposed between the titanium silicide layer and thetungsten to provide further protection and to reduce the contactresistivity of the W/TiN/TiSi₂ structure.

The blanket layer of titanium is deposited by conventional techniques,such as, by chemical vapor deposition, to a thickness of anywhere from50 to 500 angstroms. However, it is preferred that a layer be as thin aspossible and on the order of 50 to 200 angstroms.

Next, as shown in FIG. 3b, the substrate is annealed at approximately670° C. for 30 minutes in a nitrogen atmosphere causing the underlyingsilicon in regions 72, 74 and 76 to react with the titanium in layer 88to form TiSi₂ in regions 90, 92 and 94 with TiNxOy on the surface. Thetitanium layer 88 on the silicon dioxide regions 80, 82, 84 and 86 doesnot form silicide, but forms TiNxOy which can be selectively removed.The values x and y can take on any value between 0 and 1.

Next, as shown in FIG. 3c, the unnecessary titanium layer 88 on therecessed oxide regions 84 and 86 and on the sidewall spacers 80 and 82is removed selectively by etching. An etching solution of a hydrogenperoxide and sulphuric acid type can be used as the etching solution forthis purpose. This type of etching solution does not act upon thetitanium silicide layers 90, 92 and 94. Annealing at a temperature above700° may result in the formation of TiSiO on the oxide regions whichmust be removed by using an aqueous solution of hydroflouric acid.However, if the initial anneal is performed at 700° C. or less, thesilicide formation over the oxide is minimal and can be etched off withthe hydrogen peroxide and sulphuric acid solution.

Thereafter, as shown in FIG. 3d, annealing is performed at approximately800° C. or above for 30 minutes in a nitroqen atmosphere which bothconverts the titanium silicide to TiSi₂ and converts the surface of thetitanium silicide layer to titanium nitride. To increase the thicknessof TiN, TiSi₂ is annealed in NH₃ which is very reactive compared to N₂.Also TiSi₂ formed at 675° C., for 30 minutes in N₂ can be directlysubjected to an NH₃ anneal at 675° C., for 30 minutes. Thus, the sametemperature at which the first anneal is done can be used for the NH₃anneal. Thus, the resulting barrier layers comprise lower layers 90, 92and 94 of TiSi₂ and upper layers 96, 98 and 100 of TiN. As a result ofthe high temperature annealing, the resistivity of the titanium silicidelayer is substantially reduced to a very low value. The sheet resistanceof the film is approximately 6-10 ohm/sq. Although the titanium nitridelayer has a sheet resistance greater than the titanium silicide layer,the relative sheet resistance of the entire barrier layer issignificantly reduced. Thus, the two layer barrier layer is formed in aself-aligned manner without the need for photolithographic masks. Asshown in FIG. 3e, the selective deposition of the refractory metal suchas tungsten is carried out in accordance with the method described abovein the present invention to form source, drain and gate contacts 102,104 and 106.

In the silane reduction step of the invention for selectively depositingtungsten, the WF₆ and the SiH₄ flow rates are in the range between20-300 SSCM, with the criteria that the ratio of SiH be less than oneand preferably approximately 0.4 to 0.6. The temperature as mentionedabove is in the range 370° to 550° C. and the pressure is in the rangefrom 100 to 500 mT. For the hydrogen reduction step, the H₂ flow rate isbetween 3,000 to 4,000 SSCM and the WF₆ flow rate is between 200 to 300SSCM. The pressure is between 100 to 2,000 mT and the temperature isapproximately 550° C. In one specific example of the silane reductionstep, WF₆ was first introduced at a flow rate of 200 SSCM and SiH₄ wassubsequently introduced at a flow rate of 90 SSCM resulting in a SiH₄/WF₆ ratio of 0.45. The temperature was 480° C. and the pressure was 200mT. Approximately, 300 nanometers of tungsten was uniformly depositedselectively on a TiN/TiSi₂ barrier with a growth rate of approximately60 nm/min. The sheet resistance was approximately 0.3 ohm/sq and theresistivity was approximately 9 μohm-cm.

In a second example, after the deposition of 100 nm of tungsten inaccordance with the first example, the flow of SiH₄ was stopped and H₂was introduced at a flow rate of 4,000 SSCM. The temperature was thenincreased up to 550° C. and the pressure was 400 mT. The depositionprocess took place in approximately 30 minutes and resulted in a totalamount of approximately 1200 nm of tungsten uniformly deposited on theTiN with a growth rate of around 40 nm/min. The sheet resistance wasabout 0.065 to 0.08 ohm/sq. The resistivity was approximately 8-9μohm-cm.

FIG. 4 shows the change in the resistivity of the tungsten layer ofapproximately 150 nm as a function of the turret temperature. It hasbeen determined that the ideal range of the turret temperature isapproximately between 430° and 480° C., as temperatures above thatresult in poor selectivity of the deposition while temperatures belowthat begin to result in increased resistivity, poor adhesion and highstress. Thus, the method of the invention provides an excellent tungstenfilm having a resistivity of less than 17 μohm-cm. FIG. 5 shows thechange in contact resistance as the size of the contact is increased.Several diode junctions were formed having a W/TiN/TiSi₂ contact. Thegraph of FIG. 5 shows that as the contact size increased from 0.6 to 1.0μm, the contact resistance decreased from 17 to 10 ohms.

FIG. 6 is a plot showing stress as a function of turret temperatureindicating that the stress significantly increases as the temperature isreduced. Thus, for the production of a low stress film with goodselectivity, the deposition process should take place in hightemperatures, which includes the broad range between 370° and 550° C.and preferably between 430° and 480° C. The process according to thepresent invention deposits tungsten on self-aligned TiN/TiSi₂ whichenhances the conductivity of the source, drain and gate contacts forsubmicron semiconductor devices without degrading junctions in the gateoxide. In addition, the nitride layer acts as a barrier against fluorineattack during the tungsten deposition steps.

While the invention has been particularly shown and described withrespect to illustrative and preferred embodiments thereof, it will beunderstood by those skilled in the art that the foregoing and otherchanges in form and details may be made therein without departing fromthe spirit and scope of the invention which should be limited only bythe scope of the appended claims.

Having thus described the invention, what is claimed as new and what isdesired to be secured by Letters Patent is:
 1. A method for selectivelydepositing by silane reduction a refractory metal layer on exposedsurfaces of a silicon substrate having oxide regions thereon and locatedin a deposition apparatus, the method comprising the step of introducinga mixture of silane gas and a refraqtory metal halide gas into saiddeposition apparatus at a ratio of the flow rates of the silane withrespect to the refractory metal halide of less, than one and atdeposition temperatures sufficiently high such that said gases react toselectively deposit said refractory metal layer on said exposed siliconsubstrate with a resistivity of less than 17 μohm-cm.
 2. The method ofclaim 1 wherein the deposition temperatures are in the range of 370° to550° C.
 3. The method of claim 1 wherein the deposition temperatures arein the range 430° to 480° C.
 4. The method of claim 1 wherein therefractory metal halide gas is a refractory metal pentahalide gas or arefractory metal hexahalide gas.
 5. The method of claim 1 or 4 whereinthe refractory metal halide gas is a refractory metal chloride gas or arefractory metal fluoride gas.
 6. The method of claim 1 wherein therefractory metal layer comprises a refractory metal selected from thegroup consisting of tungsten, molybdenum and tantalum.
 7. The method ofclaim 6 further including the step of depositing a metal barrier layeron said exposed silicon surface prior to the deposition of saidrefractory metal layer.
 8. The method of claim 7 wherein said barrierlayer comprises a refractory metal silicide.
 9. The method of claim 7wherein said barrier layer comprises a refractory metal silicide lowerlayer and a refractory metal nitride upper layer.
 10. The method ofclaim 9 wherein said barrier layer is formed by depositing aself-aligned refractory metal silicide layer and subsequently convertingthe surface of said silicide layer to a refractory metal nitride layer.11. The method of claim 10 wherein said self-aligned refractory metalsilicide layer is deposited by depositing a blanket layer of arefractory metal on said substrate, annealing said substrate to convertthe refractory metal disposed on said exposed silicon surfaces to arefractory metal silicide, and etching away the unconverted portions ofsaid refractory metal.
 12. The method of claim 8 wherein said refractorymetal barrier layer comprises a silicide of a refractory metal selectedfrom the group consisting of titantium, niobium, zirconium, chromium andhafnium.
 13. The method of claim 9 wherein said refractory metalsilicide layer and said refractory metal nitride layer each comprises arefractory metal selected from the group consisting of titantium,niobium, zirconium, chromium and hafnium.
 14. The method of claim 7wherein said barrier layer comprises a noble metal silicide.
 15. Themethod of claim 14 wherein said noble metal silicide layer comprises anoble metal selected from the group consisting of cobalt and platinum.16. The method of claim 7 wherein said barrier layer comprises a noblemetal silicide lower layer and a refractory metal nitride upper layer.17. The method of claim 16 wherein said refractory metal nitride layercomprises a refractory metal selected from the group consisting oftitantium, niobium, zirconium, chromium and hafnium.
 18. The method ofclaim 17 wherein the noble metal silicide layer comprises a noble metalselected from the group consisting of platinum and cobalt.
 19. Themethod of claim 7 wherein said barrier layer comprises a compositerefractory metal silicide comprising two refractory metals selected fromthe group consisting of titantium, niobium, zirconium, chromium,hafnium, tungsten, molybdenum and tantalum.